1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with a means for producing sliding formants.
2. Description of the Prior Art
One of the essential subsystems contained in an electronic musical instrument of the generic type called a synthesizer, is a sliding formant generator. The sliding formant subsystem is usually implemented as a frequency filter system of either the low pass or high pass type configured so that it has the capability of varying the filter cut-off frequency in response to an electrical control signal.
The most commonly employed sliding formant generator used with analog musical tone generators is that of a voltage controlled frequency filter. This filter is used to vary the analog musical waveshape spectral response under the action of control signal. Digital tone generators can obtain the corresponding spectral variations by employing digital filters to vary the spectral content of a digital sequence of waveshape points which are later converted to analog signals to furnish the musical waveshape. A digital filter implementation for use as a formant filter subsystem is described in the copending patent application Ser. No. 839,916 filed Oct. 6, 1977, now U.S. Pat. No. 4,267,761, and entitled "Musical Tone Generator Utilizing Digital Sliding Formant Filter." This invention and the present invention have a common assignee.
As a general rule digital filters are not low cost systems because they are usually configured from basic elements having the functions of data delay, multipliers, coefficient memories, and adders. The multiplier is the most undesirable element because such digital operations are relatively expensive in comparison to the other digital logic operations comprising a digital filter.
The conventional variety of digital filters for use as sliding formant generators has been circumvented in digital tone generators of the type in which the musical waveshape is computed using a discrete Fourier transform algorithm employing preselected sets of harmonic coefficients. The selected harmonic coefficients are readily scaled in a time variant manner such that the resultant computed musical waveshape exhibits a spectral effect similar to that obtained by the sliding formant filters. Such systems have been described in U.S. Pat. No. 3,809,786 entitled "Computor Organ" and in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer."
In U.S. Pat. No. 4,130,043 entitled "Electronic Musical Instrument Having Filter-And-Delay Loop For Tone Production" a type of sliding formant filter is described which uses a single fixed filter employed in a signal feedback configuration. The musical waveshape source is a digital memory containing equally spaced points for one period of the corresponding musical waveshape. This data is sequentially and repetitively read out of the memory at the preselected clock rates and converted to an analog waveshape. In response to a command signal, the output data from the memory for a single period is passed through a low pass digital filter and used to replace the data in the memory. It was claimed that this process would produce the effect of a sliding low pass filter whose cut-off frequency varies with the number of times that data from the memory has been processed by the filter. No provision is made for sliding the formant in both an increasing and decreasing cut-off frequency excursion.